... and something my wise mentor told me. If you have to apply more force than a light "umph", something is wrong. Never apply too much force!

When I take my lenses apart, I always use a black marker and write a small dot ON THE LENS to show which side is top before taking out the lens out of place. It's really easy to accidentally flip them and not being able to tell which side is which.

If you are not comfortable with this, please don't try. It's really easy to mess it up. I only do this on lenses that if I mess up, I don't have to cry.

How do you re-collimate the lenses when you re-assemble them to ensure that the elements are centred to a datum line, and are parallel to each other ?